The present invention relates to cellular mobile telephone systems, and more particularly, to methods and devices for accommodating various designs of subscriber telephones based on different trade-offs between cost and performance.
In a cellular telephone system, capacity to support as many simultaneous conversations as possible is of the essence to the service providers because it directly impacts their revenue. At the same time, cellular telephones are a consumer product, making low cost of the essence to the retail distributors. It is possible to design systems and telephones that are capable of increasing capacity at the expense of retail cost. It is also possible to reduce the cost of such hardware at the expense of capacity. This conflict could be handled by having the service providers offer a subsidy to the retail outlet on more expensive telephones whose use they may wish to encourage for capacity reasons. This has the disadvantage, however, of restricting freedom of independent competition for equipment and services because the retail customer cannot be permitted to acquire the telephone with the maximum subsidy from a first operator and then convert the subscription to a second operator having lower subscription and call charges because he does not offer such subsidies.
One such example of a more expensive telephone that has advantages for the service provider but only disadvantages for the retail customer is the use of the diversity receiving technique. It can be shown that the provision of two separate receiving chains and antennas in a mobile telephone can reduce the signal-to-interference ratio (C/I) required for good quality communication by as much as 7 dB. Since the number of times a radio frequency channel can be re-used for separate conversations in a given geographical zone is directly related to the required C/I, the capacity of the telephone network can be approximately doubled by use of mobiles capable of diversity reception.
Because not all mobile subscribers can be expected to purchase the more expensive units, it is desirable to provide a way of accommodating mobile telephones both with and without the above exemplary improvements and corresponding frequency-reuse restrictions in the same geographical area. It is further desirable to provide the retail customer with an incentive to acquire the more complex product, since this permits the mobile communications system to operate at increased capacity.